In the field of orthopedic surgery, ZIMALOY manufactured by Zimmer, U.S.A. Inc., a chromium-cobalt-molybdenum alloy, stainless steel, titanium alloys, and polymerized materials such as ultra high molecular weight polyethylene (hereinafter UHMWPE) have been used successfully to replace the ends of long bones and joints, including the hip joint. However, there exists a severe limitation with respect to such orthopedic surgery, namely, coupling of the prosthesis to bone. Due to such factors as mechanical stress, fatigue, corrosion, etc., the prosthesis/bone cement joints have been prone to failure.
Present methods of utilizing such bone prosthesis involve the use of a prosthesis having a stem portion which is inserted into the interior of a bone. A bone cement comprising a mixture of polymethylmethacrylate (“PMMA”) polymer and methyl methacrylate monomer and optionally including a styrene copolymer of PMMA is likewise inserted into the bone cavity and utilized to couple the stem of the implant to the bone itself. Experience has demonstrated, however, that serious drawbacks exist with respect to the coupling between the prosthesis stem and the bone cement. Attempted solutions to this problem have been directed primarily toward strengthening the prosthesis/bone cement interface by means of gross mechanical interlock involving, for example, dove tails, small stems, and the like. Such devices result in stress concentrations that can exceed the strength of the bone cement and that can cause non-physiological force distribution in the bone.
Adherence at the interface between the implant and PMMA is greatly restricted by current industrial and surgical practices. For instance, the PMMA cement is typically applied in a highly viscous, doughy state with the result that the degree of contact between the implant and the cement is inadequate. Moreover, the existence of wear boundary layers such as contaminants and weak metal oxides on the surface of the implant have also caused problems. Weak boundary layers may be due to the composition of the implant or to the process of forming the same. Thus, in the case of a metal implant, the surface of the implant normally includes weak metal oxides as weak boundary layers. Finally, the implant may come in contact with air, blood or water prior to being inserted into the bone, thereby becoming contaminated. The existence of weak boundary layers, e.g., surface contaminants, is detrimental to the formation of good implant bone cement adherence. Thus, the strength of such joints has been dependent upon gross mechanical interlock. Such difficulties in the formation of a satisfactory prosthesis/bone cement connection have also made resurfacing of a deteriorated joint, e.g., a deteriorated hip joint due to arthritis, difficult to accomplish. Thus, in the case of a deteriorated articular surface, e.g., surface of the head or ball in a ball and socket joint, the entire head of the bone is generally removed and a prosthetic head is connected to the bone; although in some instances, resurfacing implants have been used with bone cement.
U.S. Pat. No. 4,336,618 to Simon Raab, which is assigned to the assignee hereof, all of the contents of which are incorporated herein by reference, taught that the aforementioned prosthesis fixation problems could be overcome by treating at least that portion of the prosthesis which is adapted to be connected to bone with a PMMA film fixedly adhered to said portions of the prosthesis. Prior to the application of the PMMA film, the surface to be coated is treated to prevent formation of a weak boundary layer upon bonding of a bone cement to an applied film. Thereafter, a PMMA film is applied by dipping, painting, spraying, etc., and finally, after the film has dried, it is annealed to remove any stresses in the film.
U.S. Pat. No. 6,136,038 to Simon Raab, which is assigned to the assignee hereof, all of the contents of which are incorporated by reference, improved upon U.S. Pat. No. 4,336,618, by lowering implant fixation failure rates, particularly those failures resulting from impact and shock conditions. U.S. Pat. No. 6,136,038 teaches that the combination of a biocompatible plasticizer with the PMMA film/cement helps to increase the resistance level of PMMA under high impact loading. The preferred plasticizer comprises the FDA approved, biocompatible hydrophilic monomer, 2-hydroxyethyl methacrylate (“HEMA”).
According to U.S. Pat. No. 6,136,038, the resultant prosthesis has a film of PMMA/HEMA firmly adhered to the surface thereof. This PMMA/HEMA film adhesively interacts molecularly with PMMA and/or PMMA/HEMA bone cement. Accordingly, the adherence of a prosthesis adhesively connected to bone by means of a PMMA or PMMA/HEMA cement can be drastically increased.
To summarize the teachings of U.S. Pat. No. 4,336,618 and U.S. Pat. No. 6,136,038 discussed hereinabove, the PMMA/HEMA film is fixedly adhered to those portions of the prosthesis which are adapted to be connected to bone. Prior to the application of the PMMA/HEMA film, the surface to be coated is treated to prevent formation of a weak boundary layer upon bonding of a bone cement to an applied film. The methods used for the application of the precoats typically include preparing the metal surface by cleaning and passivation, and then either solvent based lacquer polymerizing solutions of monomer catalyst or inhibitor, and polymer electrostatically-applying or dip-applying power coatings. In all cases, some curing and/or annealing heat cycles are used. Thereafter, a PMMA/HEMA film is applied by dipping, painting, spraying, etc., and finally, after the film has dried, it is annealed to remove any stresses in the film. The overall composition of the coating has been limited to the PMMA/HEMA composition and other standard approved inhibitors and catalysts.
Current work in the field of prosthetics has focused on improving the bonding strength between the PMMA/HEMA treated prosthetic to bone. A recurring and predominant problem associated with prostheses is debonding at the implant-cement interface. Such debonding causes cracking around the implant and through the cement mantle to the bone. In addition to increasing bonding strength, pretreatment of the prosthetic element with an agent that can increase the durability of the prosthesis in a saline environment is also desirable. A deterrent to achieving both adhesiveness and durability is the limited number of biocompatible coupling agents capable of enhancing adhesion between the bone cement and the prostheses.